The present invention, in some embodiments thereof, relates to a system, apparatus and method for circuit switch fallback (CSFB) for a long term evolution (LTE) mobile network and, more particularly, but not exclusively, to a way of providing CSFB without needing to change or upgrade existing 2G or 3G components.
LTE, otherwise known as fourth generation or 4G, is slowly being implemented in cellular telephone networks around the world. LTE is also defined to use a form of Voice Over IP (VOiP) known as Voice over LTE (VoLTE) in order to carry voice calls, so that the LTE network is entirely packet switched and does away with circuit switching altogether.
However the underlying networks do not necessarily yet support VoLTE, or may be slow to implement VoLTE due to the small proportion of subscribers currently having LTE-enabled handsets.
Thus, when deploying an LTE (4G) network, the mobile network operator (MNO) (mobile operator) has two options in general, related to voice call support. First of all it is possible to directly implement VoLTE (Voice over LTE). However alternatively it is possible to use circuit switch fallback (CSFB) to cause the network to take the signals from the phone and treat them as 3G/GSM/CDMA signals. Fallback may be carried out for voice calls and potentially also for SMS. Thus, whenever the handset makes a call (MO—Mobile Originated) or receives a call (MT—Mobile Terminated), the handset switches to 3G or one of the other protocols as available, using the CSFB procedure.
The CSFB requires the network mobile switching center (MSC) and the network visitor location register (VLR) elements to upgrade to support LTE, so they can communicate with the mobile management entity (MME). The MME is an LTE entity which manages the mobility of the device, and in the case of fallback triggers the MSC to register the device for 3G operation as well. The device is normally in 4G mode and there is no radio connection to the 3G components.
The fallback issue applies as much to mobile telephones located in their own networks, as to mobile telephones roaming in other networks. As will be explained below, however, some of the present solutions cause additional problems for roaming users.
Referring now to FIG. 1, mobile network 2 includes a 2G/3G track 4 with GSM compatible components, and an LTE track 6 with LTE components. An LTE enabled handset 10 registers at MME 12, which is the LTE equivalent of the MSC included in the mobile switching server MSS 14, where the 3G and 2G handsets register. The LTE handset places a call using CSFB, since it does not support VoLTE. The MME has to register the handset via a secondary registration at the MSC and then the call is forwarded as a regular circuit-switched call carried out over the 3G infrastructure.
The LTE track 6 uses Internet Protocol IP to communicate, and operates via the Internet, shown as 16. The 2G/3G track 4 uses an operator IP network and uses a combination of protocols including GSM (or CDMA) and GPRS to communicate.
Nokia Corp. has implemented a CSFB solution in a single box, referred to as the MSC Overlay that eliminates the need to upgrade the MSCs. However the MSC overlay solution suffers from the disadvantage that it is not scalable, and cannot serve a large number of subscribers. This is due to the fact that it needs to receive the actual mobile terminated calls, which it does via ISUP IAM, and this significantly limits the capacity of the MSC overlay solution. In the MSC overlay solution a single unit talks to the MMEs and acts as a fallback MSC for the entire network. In this way there is no need to upgrade individual MSCs.
Another LTE issue, which is related to CSFB as well, is the misalignment between the 4G MMEs and 3G MSCs coverage areas.
As shown in FIG. 2, there are two pool areas, pool area 1 and pool area 2, each area having its own MSC and its own MME. Pool area 1 has MSC1 and MME1, and pool area 2 has MSC2 and MME2. Each pool area is covered by a set of GSM coverage areas, LA1, LA2, LA3, and by a set of LTE coverage areas TA1, TA2, TA3. Handset 22 is located in the boundary region between the two pool areas.
While an incoming call arrives at MSC1, MSC1 sends a paging signal to MME1, either knowing or suspecting that an LTE handset is involved. The MME pages the local TA's for the phone. The handset falls back to 3G, but because the handset is located on the coverage boundary the 3G communication is equally likely to be picked up by MSC2. The result is a need for a re-registration of the device to MSC2, during an incoming voice call.
In general boundaries are fuzzy, especially in urban areas. The handset may move to MME2 but with no guarantee as to when it will be registered with MSC2, because the coverage areas do not overlap by 100%.
The mismatch leads to a roaming retry, and the call cannot be connected until a match is made.
Referring to FIG. 3, one reason for the misalignment is that the LTE areas may be of different size than the 3G areas. Here the LTE areas are shown as smaller, but the opposite may be true.
As shown in FIG. 3, the LTE regions do not line up with the 3G regions. Arrow 30 illustrates the movement of the handset between different MSC servers. If CSFB switching is carried out from one LTE cell in one TA to 3G cell in another LA where the current handset is not registered, a new location area update (LAU) must be carried out prior to executing the connection setup. The LAU may add a one to two second delay depending on the network loading.
Returning to the case of re-registration to another MSC during a call, the LAU and an update to the home location register (HLR) has to occur between different MSC servers prior to connection setup. Such a mobile terminated roaming retry procedure can add up to four or five seconds delay to the call set up time, depending on network loads.
FIG. 4 illustrates the mismatch and roaming retry procedure with the new LAU. The handset is initially paged at the old MSC which finds nothing and times out. When paging finds the handset at the new MSC, an authentication procedure and update location is carried out. The procedure connects the call to the handset but the delay is noticeable. The handset is paged from the old MSC to go to fallback, and then the old MSC notices that the phone is not there. Had the handset been at the old MSC the fallback would have worked to connect correctly. However as the handset is not to be found in the old MSC, the location update reaches a new MSC. The handset now has to do authentication and the call is passed to the new location.
At this point the old MSC knows about the new MSC, and sends the call back to the gateway MSC (GMSC) to look again for the new MSC. The call is connected, but at the cost of some considerable delay experienced by the caller until a connection is eventually made.
In roaming the delay is worse since the connection between the MSC and the GMSC is an international connection.
A theoretical solution is provided in the standard and provides for full communication between the VPMN and the HPMN via an interface known as the E interface. However the E interface is not implemented on any currently known products.
An alternative solution, known as MTRF, is shown in FIG. 5. In this case, the new MSC, upon paging the LTE handset, requests the call to be passed from the old MSC directly to the new MSC. At the same time the new MSC sends an update to the HLR, and tells the HLR that it supports MTRF. The HLR receives the update and instructs the old MSC to pass the call to the new MSC.
The MTRF solution requires integration at both sides, the visited side and the home side and in the case of roaming, the HPMN side and the VPMN side, which are different networks, so it is unlikely to be applicable to random combinations of home and visited networks, which is what most roaming consists of.
In summary, roaming retry requires a second location update, causing considerable delay, especially in roaming. Roaming retry, and MTRF both require integration at both the VPLMN and the HPLMN.